1. Field of the Invention
This invention relates to the thermal cracking of acid containing hydrocarbon feedstocks using a vaporization unit in combination with at least one thermal cracking furnace.
2. Description of the Prior Art
Thermal cracking (pyrolysis) of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes.
Basically, a hydrocarbon containing feedstock is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. The steam/hydrocarbon mixture is preheated in the convection zone of the furnace to from about 900 to about 1,000 degrees Fahrenheit (F.), and then enters the reaction (radiant) zone where it is very quickly heated to a severe hydrocarbon thermal cracking temperature in the range of from about 1,400 to about 1,550 F. Thermal cracking is accomplished without the aid of any catalyst.
This process is carried out in a pyrolysis furnace (steam cracker) at pressures in the reaction zone ranging from about 10 to about 30 psig. Pyrolysis furnaces have internally thereof a convection section (zone) and a separate radiant section (zone). Preheating functions are primarily accomplished in the convection section, while severe cracking mostly occurs in the radiant section.
After thermal cracking, depending on the nature of the primary feed to the pyrolysis furnace, the effluent from that furnace can contain gaseous hydrocarbons of great variety, e.g., from one to thirty-five carbon atoms per molecule. These gaseous hydrocarbons can be saturated, monounsaturated, and polyunsaturated, and can be aliphatic, alicyclics, and/or aromatic. The cracked gas can also contain significant amounts of molecular hydrogen (hydrogen).
The cracked product is then further processed in the olefin production plant to produce, as products of the plant, various separate individual streams of high purity such as hydrogen, ethylene, propylene, mixed hydrocarbons having four carbon atoms per molecule, fuel oil, and pyrolysis gasoline. Each separate individual stream aforesaid is a valuable commercial product in its own right. Thus, an olefin production plant currently takes a part (fraction) of a whole crude stream or condensate, and generates there from a plurality of separate, valuable products.
Thermal cracking came into use in 1913, and was first applied to gaseous ethane as the primary feed to the cracking furnace for the purpose of making ethylene. Since that time the industry has evolved to using heavier and more complex hydrocarbonaceous gaseous and/or liquid feeds as the primary feed for the cracking furnace. Such feeds can now employ a fraction of whole crude or condensate which is essentially totally vaporized while thermally cracking same. The cracked product can contain, for example, about 1 weight percent (wt. %) hydrogen, about 10 wt. % methane, about 25 wt. % ethylene, and about 17 wt. % propylene, all wt. % being based on the total weight of that product, with the remainder consisting mostly of other hydrocarbon molecules having from 4 to 35 carbon atoms per molecule.
Natural gas and whole crude oil(s) were formed naturally in a number of subterranean geologic formations (formations) of widely varying porosities. Many of these formations were capped by impervious layers of rock. Natural gas and whole crude oil (crude oil) also accumulated in various stratigraphic traps below the earth's surface. Vast amounts of both natural gas and/or crude oil were thus collected to form hydrocarbon bearing formations at varying depths below the earth's surface. Much of this natural gas was in close physical contact with crude oil, and, therefore, absorbed a number of lighter molecules from the crude oil.
When a well bore is drilled into the earth and pierces one or more of such hydrocarbon bearing formations, natural gas and/or crude oil can be recovered through that well bore to the earth's surface.
The terms “whole crude oil” and “crude oil” as used herein means liquid (at normally prevailing conditions of temperature and pressure at the earth's surface) crude oil as it issues from a wellhead separate from any natural gas that may be present, and excepting any treatment such crude oil may receive to render it acceptable for transport to a crude oil refinery and/or conventional distillation in such a refinery. This treatment would include such steps as desalting. Thus, it is crude oil that is suitable for distillation or other fractionation in a refinery, but which has not undergone any such distillation or fractionation. It could include, but does not necessarily always include, non-boiling entities such as asphaltenes or tar. As such, it is difficult if not impossible to provide a boiling range for whole crude oil. Accordingly, whole crude oil could be one or more crude oils straight from an oil field pipeline and/or conventional crude oil storage facility, as availability dictates, without any prior fractionation thereof.
Natural gas, like crude oil, can vary widely in its composition as produced to the earth's surface, but generally contains a significant amount, most often a major amount, i.e., greater than about 50 weight percent (wt. %), methane. Natural gas often also carries minor amounts (less than about 50 wt. %), often less than about 20 wt. %, of one or more of ethane, propane, butane, nitrogen, carbon dioxide, hydrogen sulfide, and the like. Many, but not all, natural gas streams as produced from the earth can contain minor amounts (less than about 50 wt. %), often less than about 20 wt. %, of hydrocarbons having from 5 to 12, inclusive, carbon atoms per molecule (C5 to C12) that are not normally gaseous at generally prevailing ambient atmospheric conditions of temperature and pressure at the earth's surface, and that can condense out of the natural gas once it is produced to the earth's surface. All wt. % are based on the total weight of the natural gas stream in question.
When various natural gas streams are produced to the earth's surface, a hydrocarbon composition often naturally condenses out of the thus produced natural gas stream under the then prevailing conditions of temperature and pressure at the earth's surface where that stream is collected. There is thus produced a normally liquid hydrocarbonaceous condensate separate from the normally gaseous natural gas under the same prevailing conditions. The normally gaseous natural gas can contain methane, ethane, propane, and butane. The normally liquid hydrocarbon fraction that condenses from the produced natural gas stream is generally referred to as “condensate,” and generally contains molecules heavier than butane (C5 to about C20 or slightly higher). After separation from the produced natural gas, this liquid condensate fraction is processed separately from the remaining gaseous fraction that is normally referred to as natural gas.
Thus, condensate recovered from a natural gas stream as first produced to the earth's surface is not the exact same material, composition wise, as natural gas (primarily methane). Neither is it the same material, composition wise, as crude oil. Condensate occupies a niche between normally gaseous natural gas and normally liquid whole crude oil. Condensate contains hydrocarbons heavier than normally gaseous natural gas, and a range of hydrocarbons that are at the lightest end of whole crude oil.
Condensate, unlike crude oil, can be characterized by way of its boiling point range. Condensates normally boil in the range of from about 100 to about 650 F. With this boiling range, condensates contain a wide variety of hydrocarbonaceous materials. These materials can include compounds that make up fractions that are commonly referred to as naphtha, kerosene, diesel fuel(s), and gas oil (fuel oil, furnace oil, heating oil, and the like).
Atmospheric residuum (“resid,” “residua”) obtained from a conventional atmospheric thermal distillation tower can have a wide boiling range, particularly when mixtures of residua are employed, but will generally be in a boiling range of from about 600 F to the boiling end point where only non-boiling entities remain. These resids are primarily composed of a gas oil component boiling in the range of from about 600 to about 1,000 F and a heavier fraction boiling in a temperature range of from about 1,000 F up to its end boiling point where only non-boiling entities remain.
In contrast to an atmospheric tower, a vacuum assisted thermal distillation tower (vacuum tower) typically separates this gas oil component from its associated heavier fraction aforesaid, thus freeing the gas oil fraction for separate recovery and use elsewhere.
The olefin production industry is now progressing beyond the use of fractions of crude oil or condensate (gaseous and/or liquid) as the primary feed for a cracking furnace to the use of whole crude oil, crude oil residuum, and/or condensate itself as a significant part of that feed.
U.S. Pat. No. 6,743,961 (hereafter “USP '961”) recently issued to Donald H. Powers. This patent relates to cracking whole crude oil by employing a vaporization/mild cracking zone that contains packing. This zone is operated in a manner such that the liquid phase of the whole crude that has not already been vaporized is held in that zone until cracking/vaporization of the more tenacious hydrocarbon liquid components is maximized. This allows only a minimum of solid residue formation which residue remains behind as a deposit on the packing. This residue is later burned off the packing by conventional steam air decoking, ideally during the normal furnace decoking cycle, see column 7, lines 50-58 of that patent. Thus, the second zone 9 of that patent serves as a trap for components, including hydrocarbonaceous materials, of the crude oil feed that cannot be cracked or vaporized under the conditions employed in the process, see column 8, lines 60-64 of that patent.
U.S. Pat. No. 7,019,187, issued to Donald H. Powers, is directed to the process disclosed in USP '961, but employs a mildly acidic cracking catalyst to drive the overall function of the vaporization/mild cracking unit more toward the mild cracking end of the vaporization (without prior mild cracking)—mild cracking (followed by vaporization) spectrum.
U.S. Pat. No. 7,404,889, issued to Donald H. Powers, is directed to the process disclosed in USP '961, but uses atmospheric residuum as the dominant liquid hydrocarbonaceous feed for the vaporization unit and furnace.
The disclosures of the foregoing patents, in their entirety, are incorporated herein by reference.
U.S. patent application Ser. No. 11/365,212, filed Mar. 1, 2006, having common inventorship and assignee with USP '961, is directed to the use of condensate as the dominant liquid hydrocarbonaceous feed for the vaporization unit and furnace.
U.S. Application Publication 2007/0066860 John S. Buchanan et al., published Mar. 22, 2007, discloses the thermal cracking of crudes that have a high Total Acid Number (TAN) using a flash drum unit in combination with a thermal cracking furnace. This Publication discloses that its flash drum effects only a physical separation of the two phases (vapor and liquid) entering that drum. That is to say, the composition of the vapor phase leaving the flash drum is disclosed to be substantially the same as the composition of the vapor phase entering that drum. Likewise, the composition of the liquid phase leaving the same flash drum is disclosed to be substantially the same as the composition of the liquid phase entering that drum. Preferred high TAN feeds are disclosed to be crude or a feed stream that has previously been subjected to a refinery process to remove resid. Thus, Buchanan et al. teach away from the use of resids in its process.
The Publication to Buchanan et al. further discloses that the naphthenic acids present in its high TAN feeds are substantially converted to CO, CO2, and lower molecular weight acids such as formic, acetic, propionic, and butyric acids.
Organic acids, including naphthenic acids, are present to a growing extent in hydrocarbonaceous feeds such as crude oil, and are becoming a problem for crude oil refining processors. Naphthenic acids are often singled out for consideration because they are particularly corrosive.
Most refineries are unable to process crude oils with total acid numbers (TAN) greater than 1.0 due to the highly corrosive nature of the acids, particularly naphthenic acids, above 400 F. As more and more of the World's hydrocarbon production capacity is required to meet demand, the use of these acid containing feedstocks, particularly crude oils, is required to meet worldwide demand growth.
By this invention, organic acid containing feedstocks such as whole crude oil, and condensate, and organic acid containing fractions of crude oil, e.g., residua, are processed by a combination of a vaporization unit and at least one thermal cracking furnace not only to reduce (convert or transform) the original acid content, but also to form additional thermal cracking feed from those feedstocks.